The invention relates to a converter for an ion propulsion system.
Ion propulsion systems require electrical voltages in a range of several thousand volts in order to operate. Therefore, there is a need for power supplies that can achieve a high specific output and simultaneously exhibit high efficiency. In order to generate high voltages, the power supplies comprise converters that should exhibit, if possible, only a single converter stage in order to achieve the requisite high specific output while simultaneously continuing to exhibit high efficiency. If, however, a pre-regulator is dispensed with, it must be possible to control the single stage converter in its entirety. That is, when running with no load as well as under maximum load, the output voltage has to stay within the allowable limits.
In this case the propulsion systems do not constitute a purely resistive load. In gridless ion propulsion systems plasma variations may lead to periodic changes in the impedance. Gridded ion propulsion systems may develop discharges that result in transient shorts (beam out). Therefore, the anode current supply has to be insensitive to changes in the impedance of the propulsion system. Furthermore, it is generally necessary to electrically isolate the low voltage side from the high voltage side. As a result, a transformer has to be used. However, the high transformation makes it necessary to take the winding capacitance of said transformer into special consideration.
The prior art converters for ion propulsion systems employ two stages. The first stage comprises a pre-regulator, which does not exhibit electrical isolation and which generates a variable intermediate circuit voltage and a variable intermediate circuit current. The second stage is an unregulated stage for electrical isolation and for raising to the required voltage value. The unregulated stage is designed as a full resonant mode bridge circuit or as a push-pull stage. In both cases zero voltage switches and zero current switches (ZVS: zero voltage switching and ZCS: zero current switching) are used.
The publication “Isolated DC-DC Converters With High-Output Voltage for TWTA Telecommunication Satellite Applications,” Ivo Barbi, Roger Gules, IEEE Transactions on Power Electronics, Vol. 18, No. 4, July 2003, introduces a single stage design as a further development of the pull-push stage. In this case, however, the large storage choke of the prestage is maintained.
Another single stage design, which is based on a three phase, fully resonant converter, is introduced in the publication “Ion and Plasma Thruster Test Console Based on Three-Phase Resonant Conversion Power Modules,” Geoffrey N. Drummond, John D. Williams, 42nd AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, Jul. 9-12, 2006, Sacramento, Calif. However, full resonant mode converters can be controlled only by detuning the switching frequency within certain limits.
In the publication “A New High-Efficiency Zero-Voltage, Zero-Current Switching Topology,” S. H. Weinberg, European Space Agency, TEC-EPC, Postbus 299, 2200AG Noordwijk, The Netherlands, a push-pull stage with quasi resonant switching is used as the main transformer and operates with ZVS/ZCS. In order to expand the control range, a smaller converter of the same type varies the primary voltage of the main converter. As a result, the ZVS/ZCS operating mode is maintained over a larger adjustment range.
The common feature of all of the solutions lies in the compatibility with the winding capacitance of the high voltage transformer. Therefore, designs exhibiting a secondary-sided current smoothing are eliminated.
Exemplary embodiments of the present invention provide an improved converter, which can be employed for ion propulsion systems. In particular, the present invention converts the voltage with a converter. The present invention permits a high transformation ratio and, nevertheless, offers a simple and, thus, easy as well as economical possibility for adjusting the load current without having to be concerned about a high ripple of the output voltage.
The present invention is based on the recognition that it is possible to achieve a conversion of the voltage using a precise, but simultaneously simple adjustment of the load current in that the necessary circuit engineering measures are implemented in the primary and secondary circuit of the converter, and that the (high) capacitance of the secondary winding, necessitated by the high transformation ratio, cannot exert any negative influence on the voltage conversion. In addition, a time delayed opening and/or closing of the switches inside the bridge circuit allows the resulting current paths to divide cyclically and conjoin again and the coupling point to be adjusted using the time lag in opening and/or closing the switches of the bridge circuit. Since, however, this coupling point acts directly on the current amplitude, which occurs during the steady state mode of the voltage conversion, the amount of the high voltage generated during the voltage conversion can also be indirectly controlled by adjusting the coupling point.
Thus, the present invention offers the advantage that, in contrast to the state of the art, simple engineering means make possible a good controllability of the load current while simultaneously continuing to exhibit a high voltage transformation ratio. Moreover, at the same time, in addition, a reaction to the impedance variations in the supplied load can still ensue with an adequate matching to the power to be transmitted. Furthermore, the matching to the power to be transmitted is realized only in a recharging operation during a switch-over operation of the converter so that there is almost no need to change the pulse duty factor. Thus, the required ripple of the current, which is consumed by the converter, and the required ripple of the load current can be achieved with relatively low filtering complexity, even though the switching frequency can be set low (a feature that helps to reduce the frequency dependent losses in the filtering components or storage chokes).
One embodiment of the invention provides a converter for an ion propulsion system. The converter includes a bridge circuit with a first and second bridge circuit connection and with two bridge branches, the bridge circuit comprising four switches, each having a first and second connection, the first connection of a first and third switch respectively being connected to the first bridge circuit connection; the second connection of the first switch being connected to the first connection of a second switch; the second connection of the third switch being connected to the first connection of a fourth switch; and the second connection of the second and fourth switch respectively being connected to the second bridge circuit connection.
The converter also includes a storage inductance and a high voltage transformer with a primary and a secondary winding, the primary winding and the storage inductance being connected in series; and this series connection being connected to the second connection of the first switch and the second connection of the third switch; and the secondary winding being connected to an output of the converter.
The converter further includes a switch control unit, which is designed to drive switches inside a bridge branch in such a manner that a delay time is inserted between the switch-over process in order to prevent a bridge short circuit and in order to be able to carry out the switching operations under zero voltage conditions or lowest voltage.
Furthermore, this switch control unit is designed to open or close-so as to be time delayed in relation to a switch of the second bridge branch-a switch of the first bridge branch at a control time.
According to a further development of the invention, an auxiliary inductance is connected between the primary winding and the second connection of the third switch.
As an alternative or in addition to providing the auxiliary inductance, a further embodiment of the invention may provide the converter with a diode branch, comprising two diodes. In this case the cathode of the first diode is connected between the primary winding of the high voltage transformer of the auxiliary inductance; and the anode of the first diode is connected to the second bridge circuit connection. The cathode of the second diode should be connected to the first bridge circuit connection; and the anode of the second diode should also be connected between the primary winding and the auxiliary inductance. This embodiment is provided for the case, where MOSFET transistors are used as the switches. Then the body diode of these switches is not operated in the hard switching mode, for which reason it is possible to avoid a failure of the converter.
Furthermore, in one embodiment of the invention, one capacitance respectively may also be connected between the first and second connections of the second and fourth switch. As a result, the voltage load can be kept advantageously low at all four switches during the turn-on operation (zero voltage switching), so that the turn-on losses may be kept as low as possible.
Furthermore, in another embodiment of the invention, one diode respectively may be connected between the first and second connections of all switches, if the switch design that is used does not exhibit any inherent diode, the anode of which is connected to the first connection and the cathode of which is connected to the second connection of the respective switch.
In another embodiment of the present invention, a capacitance may be connected in parallel to the secondary winding of the high voltage transformer. This feature makes it possible to avoid undershooting a secondary-sided minimum capacitance, which is necessary for a resonant recharging of the inductances and for the intermediate accumulation of energy.
According to another embodiment of the invention, the switch control unit may be designed to determine the control time in such a manner that it corresponds to no more than one-tenth of the period of time that elapses between an opening and closing of the same switch. This feature guarantees that the pulse duty factor is as high as possible and, moreover, does not change significantly so that the high voltage transformer generates almost square wave current pulses, so that the current waveform, resulting after rectifying, exhibits an advantageously low harmonic factor and an advantageously short gap time. Therefore, the harmonic factor is relevant for the resistive losses; and the gap time is relevant for the current ripple and the associated filtering complexity.
Furthermore, another embodiment of the invention also provides a switch control unit that is designed to use—in the event of a repeated opening of the fourth switch upon opening a first switch—a control time that is different from the preceding opening of the fourth switch. This feature makes it possible to use different control times when the converter is in operation. Therefore, these different control times also make it possible to transmit different power outputs by way of the converter. Other advantages and possible applications of the present invention may be derived from the following description in conjunction with the embodiments, depicted in the drawings.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.